Differential line drivers, such as those used within Controller Area Network (CAN) devices, typically drive unshielded twisted pair (UTP) transmission lines, which connect Electronic Control Units (ECUs) and the like to, for example, modules containing sensors or other ECUs. One requirement of such differential line drivers used within automotive applications is that they generate a low level of electromagnetic emission (EME).
In order to achieve a differential signal driver capable of generating the required low EME, one needs to make the generated signals as differential as possible. In addition, one needs to minimize power supply noise, and in particular on chip digital power supply noise, which is coupled through the final switch's gate drain capacitance to the output. One way to achieve this is using common mode feedback circuits. However, such closed-loop feedback circuits are significantly disturbed during EME (or electromagnetic interference (EMI)) events. To avoid such issues using common mode feedback, it is known to implement an open-loop driver cell with high side and low side switches, in which multiple parallel driver cells are switched ‘on’ and ‘off’ after small delays. Such a multiple parallel driver cell implementation provides a quiet common mode voltage level and therefore low EME.
A further requirement of such differential line drivers used within automotive applications is to reduce differential ringing on the transmission lines, which is a direct result of non-optimal terminated network transmission lines. When ringing is excessive, the receiver will be triggered after some ring periods giving rise to significant delay. This requirement has lead to a demand for lowering the maximum signal derivative (max|d(VCANH−CANL)/dt|) thereby providing linear rising and falling slopes.
Multiple parallel driver cells in an open-loop arrangement, using the same parallel driver cells and equal delays, results in a ‘current thermometer’ Digital to Analogue Converter (DAC). However, such a DAC requires a virtual grounded output voltage in order to linearise the rising and falling slopes, which is not supported by this driver as the driver's output is not grounded.
One can stagger non equal cells using a weighting algorithm, such as disclosed in IEC 62228, Ed. 1.0, Integrated circuits—EMC evaluation of CAN transceivers, 2007, to achieve linear rise and fall shapes. However, this approach requires lookup tables, state machines and/or different cell designs, making impedance matching and design difficult.